A usual cooling device is constructed to have a system for cooling an object indirectly by a coolant which is circulated in a refrigeration cycle. Heretofore, as such a coolant, halogen derivatives such as chlorodifluoromethane have been commonly used. However, many of them are compounds having chlorine atoms in their structures and are considered to be related to ozone depletion, and they are gradually banned. Further, as halogen derivatives containing no chlorine, perfluorocarbons (PFC) or hydrofluorocarbons (HFC) are known. These halogen derivatives are in many cases related to global warming, and their discharge is required to be controlled.
Further, it has been studied to use ammonia, hydrocarbons, carbon dioxide, etc., as the above coolant, but their commercial use has been difficult because of safety problems such as their toxicity, flammability, corrosive, etc. or for reasons such that the operation pressure tends to be high, or the energy efficiency is poor.
For the purpose of overcoming such problems, a secondary circulation cooling system is used which comprises a primary cooling means and a secondary circulation cooling means. This is a system wherein in the first cooling means, ammonia or a hydrocarbon is used as a heat transfer medium (primary coolant), and in the secondary circulation cooling means, a heat transfer medium (secondary coolant) which is less influential over the environment and has a higher level of safety, is used, and the heat energy is exchanged in a non-contact manner between the primary coolant and the secondary coolant by a heat exchanger.
The secondary coolant is required to be excellent in the heat transfer property, flowability, anticorrosion property, stability and safety. As a conventional secondary coolant, an aqueous solution of calcium chloride, sodium chloride or the like, a glycol such as ethylene glycol or propylene glycol, an alcohol, polydimethylsiloxane, a hydrocarbon, a chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC), or a PFC, may, for example, be mentioned.
However, CFCs or HCFCs are considered to remain in the environment or to be related to ozone depletion, and they are gradually banned. PFCs have a high global warming coefficient, and their discharge is required to be controlled. On the other hand, an aqueous solution of calcium chloride or sodium chloride, a glycol or an alcohol has several problems such that it is corrosive, is not sufficiently safe or requires a large power for transportation.
As a method to solve such problems, a method of using a fluorinated ether such as C3F7OCH3 as a secondary coolant, has been proposed (Patent Document 1). Although this method is less influential over the environment, with respect to the properties as a secondary coolant, such as the pressure drop and heat transfer, it is not necessarily adequate.
Patent Document 1: JP-A-11-513738 (Claims)